1. Field of the Invention
The present invention relates to polishing and lapping methods and apparatus and, more particularly, to the mounting of material wafers to the lapping plate of a lapping machine.
2. Description of the Prior Art
THe polishing and lapping steps utilized in the processing of materials such as glass and semiconductor wafers are both time-consuming and costly operations and when used in semiconductor processing the steps result in a high percentage of rejects. During all but the final processing steps for a semiconductor wafer, the wafer is maintained at a substantial thickness to reduce the likelihood of accidental breakage during processing and particularly during probe testing of the wafer. Prior to the scribing and breaking steps for dividing the wafer into individual semiconductor die, it is essential that the wafer thickness be reduced to make scribing and breaking easier. This process is commonly known as back-lapping wherein a substantial portion of the wafer is removed, by a grinding or lapping operation, from the back-side of the wafer.
Heretofore, the wafers were mounted face down on a flat lapping plate that usually is circular with a diameter of approximately twelve inches. The wafers were held in place on the lapping plate by a standard wax mounting process wherein the lapping plate was heated to the wax melting temperature and a layer of wax was formed on the flat surface, the wafers were then placed on the heated wax and clamped in position while the lapping plate cooled and the wax hardened. The excess wax then had to be removed so that no wax extended above or on the exposed back-side of the wafers. The backside of the wafers was then ground off using a lapping slurry containing an abrasive such as Al.sub.2 O.sub.3 or diamond paste for removing the semiconductor material.
After completion of the lapping process, the lapping plate and wafers were washed to remove excess lapping slurry and the plate was again heated to melt the wax, and the wafers were then removed from the lapping plate. Due to surface tension, the wafer usually had to be pried or slid off of the plate resulting in substantial damage to the active surface of the semiconductor wafers. Any sliding of the wafer on the lapping plate tended to scratch the front surface since much of the abrasive material remains embedded in the wax. Since the semiconductor devices are already formed on the front surface of the wafer, any amount of scratching will damage the devices and it is not uncommon to experience 30 to 50% rejects as a result of the back-lap process. After the wafer is removed from the waxed plate, the wafers must be cleaned with a solvent solution.
Aside from the substantial number of rejects experienced during the lapping process, the process is also extremely expensive and time-consuming. The lapping plate must first be heated to melt the wax, then fully cooled to provide good adhesion of the wafers to the plate and then again reheated for removal of the wafers. This procedure is, of course, quite time-consuming. The cleaning operation is also becoming increasingly expensive as the cost of solvent continues to rise.
Another difficulty that was encountered in the use of wax mounting was thickness control and planarity control. Due to the thickness variations of the wax layer, the wafers would have varying thicknesses and the surfaces were not always planar.